% Copyright 2019 by Till Tantau
%
% This file may be distributed and/or modified
%
% 1. under the LaTeX Project Public License and/or
% 2. under the GNU Free Documentation License.
%
% See the file doc/generic/pgf/licenses/LICENSE for more details.


\section[Hierarchical Structures: Package, Environments, Scopes, and Styles]
        {Hierarchical Structures:\\
         Package, Environments, Scopes, and Styles}

The present section explains how your files should be structured when you use
\tikzname. On the top level, you need to include the |tikz| package. In the
main text, each graphic needs to be put in a |{tikzpicture}| environment.
Inside these environments, you can use |{scope}| environments to create
internal groups. Inside the scopes you use |\path| commands to actually draw
something. On all levels (except for the package level), graphic options can be
given that apply to everything within the environment.


\subsection{Loading the Package and the Libraries}

\begin{package}{tikz}
    This package does not have any options.

    This will automatically load the \pgfname\ and the |pgffor| package.

    \pgfname\ needs to know what \TeX\ driver you are intending to use. In most
    cases \pgfname\ is clever enough to determine the correct driver for you;
    this is true in particular if you use \LaTeX. One situation where \pgfname\
    cannot know the driver ``by itself'' is when you use plain \TeX\ or Con\TeX
    t together with |dvipdfm|. In this case, you have to write
    |\def\pgfsysdriver{pgfsys-dvipdfm.def}| \emph{before} you input |tikz.tex|.
\end{package}

\begin{command}{\usetikzlibrary\marg{list of libraries}}
    Once \tikzname\ has been loaded, you can use this command to load further
    libraries. The list of libraries should contain the names of libraries
    separated by commas. Instead of curly braces, you can also use square
    brackets, which is something Con\TeX t users will like. If you try to load
    a library a second time, nothing will happen.

    \example |\usetikzlibrary{arrows.meta}|

    The above command will load a whole bunch of extra arrow tip definitions.

    What this command does is to load the file
    |tikzlibrary|\meta{library}|.code.tex| for each \meta{library} in the
    \meta{list of libraries}. If this file does not exist, the file
    |pgflibrary|\meta{library}|.code.tex| is loaded instead. If this file also
    does not exist, an error message is printed. Thus, to write your own
    library file, all you need to do is to place a file of the appropriate name
    somewhere where \TeX\ can find it. \LaTeX, plain \TeX, and Con\TeX t users
    can then use your library.
\end{command}


\subsection{Creating a Picture}

\subsubsection{Creating a Picture Using an Environment}

The ``outermost'' scope of \tikzname\ is the |{tikzpicture}| environment. You
may give drawing commands only inside this environment, giving them outside (as
is possible in many other packages) will result in chaos.

In \tikzname, the way graphics are rendered is strongly influenced by graphic
options. For example, there is an option for setting the color used for
drawing, another for setting the color used for filling, and also more obscure
ones like the option  for setting the prefix used in the filenames of temporary
files written while plotting functions using an external program. The graphic
options are specified in \emph{key lists}, see
Section~\ref{section-graphic-options} below for details. All graphic options
are local to the |{tikzpicture}| to which they apply.

\begin{environment}{{tikzpicture}\opt{\meta{animations spec}}\opt{\oarg{options}}}
    All \tikzname\ commands should be given inside this environment, except for
    the |\tikzset| command. You cannot use graphics commands like the low-level
    command |\pgfpathmoveto| outside this environment and doing so will result
    in chaos. For \tikzname, commands like |\path| are only defined inside this
    environment, so there is little chance that you will do something wrong
    here.

    When this environment is encountered, the \meta{options} are parsed, see
    Section~\ref{section-graphic-options}. All options given here will apply to
    the whole picture. Before the options you can specify animation commands,
    provided that the |animations| library is loaded, see
    Section~\ref{section-tikz-animations} for details.

    Next, the contents of the environment is processed and the graphic commands
    therein are put into a box. Non-graphic text is suppressed as well as
    possible, but non-\pgfname\ commands inside a |{tikzpicture}| environment
    should not produce any ``output'' since this may totally scramble the
    positioning system of the backend drivers. The suppressing of normal text,
    by the way, is done by temporarily switching the font to |\nullfont|. You
    can, however, ``escape back'' to normal \TeX\ typesetting. This happens,
    for example, when you specify a node.

    At the end of the environment, \pgfname\ tries to make a good guess at the
    size of a bounding box of the graphic and then resizes the picture box such
    that the box has this size. To ``make its guess'', every time \pgfname\
    encounters a coordinate, it updates the bounding box's size such that it
    encompasses all these coordinates. This will usually give a good
    approximation of the bounding box, but will not always be accurate. First,
    the line thickness of diagonal lines is not taken into account correctly.
    Second, control points of a curve often lie far ``outside'' the curve and
    make the bounding box too large. In this case, you should use the
    |[use as bounding box]| option.

    The following key influences the baseline of the resulting picture:
    %
    \begin{key}{/tikz/baseline=\meta{dimension or coordinate or \texttt{default}} (default 0pt)}
        Normally, the lower end of the picture is put on the baseline of the
        surrounding text. For example, when you give the code
        |\tikz\draw(0,0)circle(.5ex);|, \pgfname\ will find out that the lower
        end of the picture is at $-.5\mathrm{ex} - 0.2\mathrm{pt}$ (the 0.2pt
        are half the line width, which is 0.4pt) and that the upper end is at
        $.5\mathrm{ex}+.5\mathrm{pt}$. Then, the lower end will be put on the
        baseline, resulting in the following: \tikz\draw(0,0)circle(.5ex);.

        Using this option, you can specify that the picture should be raised or
        lowered such that the height \meta{dimension} is on the baseline. For
        example, |\tikz[baseline=0pt]\draw(0,0)circle(.5ex);| yields
        \tikz[baseline=0pt]\draw(0,0)circle(.5ex); since, now, the baseline is
        on the height of the $x$-axis.

        This options is often useful for ``inlined'' graphics as in
        %
\begin{codeexample}[]
$A \mathbin{\tikz[baseline] \draw[->>] (0pt,.5ex) -- (3ex,.5ex);} B$
\end{codeexample}

        Instead of a \meta{dimension} you can also provide a coordinate in
        parentheses. Then the effect is to put the baseline on the
        $y$-coordinate that the given \meta{coordinate} has \emph{at the end of
        the picture}. This means that, at the end of the picture, the
        \meta{coordinate} is evaluated and then the baseline is set to the
        $y$-coordinate of the resulting point. This makes it easy to reference
        the $y$-coordinate of, say, the baseline of nodes.
        %
\begin{codeexample}[preamble={\usetikzlibrary{shapes.misc}}]
Hello
\tikz[baseline=(X.base)]
  \node [cross out,draw] (X) {world.};
\end{codeexample}

\begin{codeexample}[]
Top align:
\tikz[baseline=(current bounding box.north)]
  \draw (0,0) rectangle (1cm,1ex);
\end{codeexample}

        Use |baseline=default| to reset the |baseline| option to its initial
        configuration.
    \end{key}

    \begin{key}{/tikz/execute at begin picture=\meta{code}}
        This option causes \meta{code} to be executed at the beginning of the
        picture. This option must be given in the argument of the
        |{tikzpicture}| environment itself since this option will not have an
        effect otherwise. After all, the picture has already ``started'' later
        on. The effect of multiply setting this option accumulates.

        This option is mainly used in styles like the |every picture| style to
        execute certain code at the start  of a picture.
    \end{key}

    \begin{key}{/tikz/execute at end picture=\meta{code}}
        This option installs \meta{code} that will be executed at the end of
        the picture. Using this option multiple times will cause the code to
        accumulate. This option must also be given in the optional argument of
        the |{tikzpicture}| environment.
        %
\begin{codeexample}[preamble={\usetikzlibrary{backgrounds}}]
\begin{tikzpicture}[execute at end picture=%
  {
    \begin{pgfonlayer}{background}
      \path[fill=yellow,rounded corners]
        (current bounding box.south west) rectangle
        (current bounding box.north east);
    \end{pgfonlayer}
  }]
  \node at (0,0) {X};
  \node at (2,1) {Y};
\end{tikzpicture}
\end{codeexample}
    \end{key}

    All options ``end'' at the end of the picture. To set an option
    ``globally'' change the following style:
    %
    \begin{stylekey}{/tikz/every picture (initially \normalfont empty)}
        This style is installed at the beginning of each picture.
        %
\begin{codeexample}[code only]
\tikzset{every picture/.style=semithick}
\end{codeexample}
    \end{stylekey}

    Note that you should not use |\tikzset| to set options directly. For
    instance, if you want to use a line width of |1pt| by default, do not try
    to say |\tikzset{line width=1pt}| at the beginning of your document. This
    will not work since the line width is changed in many places. Instead, say
    %
\begin{codeexample}[code only]
\tikzset{every picture/.style={line width=1pt}}
\end{codeexample}
    %
    This will have the desired effect.
\end{environment}

In other \TeX\ formats, you should use the following commands instead:

\begin{plainenvironment}{{tikzpicture}\opt{\oarg{options}}}
    This is the plain \TeX\ version of the environment.
\end{plainenvironment}

\begin{contextenvironment}{{tikzpicture}\opt{\oarg{options}}}
    This is the Con\TeX t version of the environment.
\end{contextenvironment}


\subsubsection{Creating a Picture Using a Command}

The following command is an alternative to |{tikzpicture}| that is particular
useful for graphics consisting of a single or few commands.

\begin{command}{\tikz\opt{\meta{animations spec}}\opt{\oarg{options}}\marg{path commands}}
    This command places the \meta{path commands} inside a |{tikzpicture}|
    environment. The \meta{path commands} may contain paragraphs and fragile
    material (like verbatim text).

    If there is only one path command, it need not be surrounded by curly
    braces, if there are several, you need to add them (this is similar to the
    |\foreach| statement and also to the rules in programming languages like
    Java or C concerning the placement of curly braces).

    \example |\tikz{\draw (0,0) rectangle (2ex,1ex);}| yields
    \tikz{\draw (0,0) rectangle (2ex,1ex);}

    \example |\tikz \draw (0,0) rectangle (2ex,1ex);| yields
    \tikz \draw (0,0) rectangle (2ex,1ex);
\end{command}


\subsubsection{Handling Catcodes and the Babel Package}

Inside a \tikzname\ picture, most symbols need to have the category code 12
(normal text) in order to ensure that the parser works properly. This is
typically not the case when packages like |babel| are used, which change
catcodes aggressively.

To solve this problem, \tikzname\ provides a small library also called |babel|
(which can, however, also be used together with any other package that globally
changes category codes). What it does is to reset the category codes at the
beginning of every |{tikzpicture}| and to restore them at the beginning of
every node. In almost all cases, this is exactly what you would expect and
need, so I recommend to always load this library by saying
|\usetikzlibrary{babel}|. For details on what, exactly, happens with the
category codes, see Section~\ref{section-library-babel}.


\subsubsection{Adding a Background}

By default, pictures do not have any background, that is, they are
``transparent'' on all parts on which you do not draw anything. You may instead
wish to have a colored background behind your picture or a black frame around
it or lines above and below it or some other kind of decoration.

Since backgrounds are often not needed at all, the definition of styles for
adding backgrounds has been put in the library package |backgrounds|. This
package is documented in Section~\ref{section-tikz-backgrounds}.


\subsection{Using Scopes to Structure a Picture}

Inside a |{tikzpicture}| environment you can create scopes using the |{scope}|
environment. This environment is available only inside the |{tikzpicture}|
environment, so once more, there is little chance of doing anything wrong.


\subsubsection{The Scope Environment}

\begin{environment}{{scope}\opt{\meta{animations spec}}\opt{\oarg{options}}}
    All \meta{options} are local to the \meta{environment contents}.
    Furthermore, the clipping path is also local to the environment, that is,
    any clipping done inside the environment ``ends'' at its end.
    %
\begin{codeexample}[]
\begin{tikzpicture}[ultra thick]
  \begin{scope}[red]
    \draw (0mm,10mm) -- (10mm,10mm);
    \draw (0mm,8mm) -- (10mm,8mm);
  \end{scope}
  \draw (0mm,6mm) -- (10mm,6mm);
  \begin{scope}[green]
    \draw (0mm,4mm) -- (10mm,4mm);
    \draw (0mm,2mm) -- (10mm,2mm);
    \draw[blue] (0mm,0mm) -- (10mm,0mm);
  \end{scope}
\end{tikzpicture}
\end{codeexample}

    \begin{key}{/tikz/name=\meta{scope name}}
        Assigns a name to a scope reference in animations. The name is a
        ``high-level'' name that drivers do not see, so you can use spaces,
        number, letters, in a name, but you should \emph{not} use any
        punctuation like a dot, a comma, or a colon.
    \end{key}

    The following style influences scopes:
    %
    \begin{stylekey}{/tikz/every scope (initially \normalfont empty)}
        This style is installed at the beginning of every scope.
    \end{stylekey}

    The following options are useful for scopes:
    %
    \begin{key}{/tikz/execute at begin scope=\meta{code}}
        This option install some code that will be executed at the beginning of
        the scope. This option must be given in the argument of the |{scope}|
        environment.

        The effect applies only to the current scope, not to subscopes.
    \end{key}

    \begin{key}{/tikz/execute at end scope=\meta{code}}
        This option installs some code that will be executed at the end of the
        current scope. Using this option multiple times will cause the code to
        accumulate. This option must also be given in the optional argument of
        the |{scope}| environment.

        Again, the effect applies only to the current scope, not to subscopes.
    \end{key}
\end{environment}

\begin{plainenvironment}{{scope}\opt{\meta{animations spec}}\opt{\oarg{options}}}
    Plain \TeX\ version of the environment.
\end{plainenvironment}

\begin{contextenvironment}{{scope}\opt{\meta{animations spec}}\opt{\oarg{options}}}
    Con\TeX t version of the environment.
\end{contextenvironment}


\subsubsection{Shorthand for Scope Environments}

There is a small library that makes using scopes a bit easier:
%
\begin{tikzlibrary}{scopes}
    This library defines a shorthand for starting and ending |{scope}|
    environments.
\end{tikzlibrary}

When this library is loaded, the following happens: At certain places inside a
\tikzname\ picture, it is allowed to start a scope just using a single brace,
provided the single brace is followed by options in square brackets:
%
\begin{codeexample}[preamble={\usetikzlibrary{scopes}}]
\begin{tikzpicture}
  { [ultra thick]
    { [red]
      \draw (0mm,10mm) -- (10mm,10mm);
      \draw (0mm,8mm)  -- (10mm,8mm);
    }
    \draw (0mm,6mm) -- (10mm,6mm);
  }
  { [green]
    \draw (0mm,4mm) -- (10mm,4mm);
    \draw (0mm,2mm) -- (10mm,2mm);
    \draw[blue] (0mm,0mm) -- (10mm,0mm);
  }
\end{tikzpicture}
\end{codeexample}

In the above example, |{ [ultra thick]| actually causes a
|\begin{scope}[ultra thick]| to be inserted, and the corresponding closing |}|
causes an |\end{scope}| to be inserted.

The ``certain places'' where an opening brace has this special meaning are the
following: First, right after the semicolon that ends a path. Second, right
after the end of a scope. Third, right at the beginning of a scope, which
includes the beginning of a picture. Also note that some square bracket must
follow, otherwise the brace is treated as a normal \TeX\ scope.


\subsubsection{Single Command Scopes}

In some situations it is useful to create a scope for a single command. For
instance, when you wish to use algorithm graph drawing in order to layout a
tree, the path of the tree needs to be surrounded by a scope whose only purpose
is to take a key that selects a layout for the scope. Similarly, in order to
put something on a background layer, a scope needs to be created. In such
cases, where it will cumbersome to create a |\begin{scope}| and |\end{scope}|
pair just for a single command, the |\scoped| command may be useful:

\begin{command}{\scoped\opt{\meta{animations spec}}\opt{\oarg{options}}\meta{path command}}
    This command works like |\tikz|, only you can use it inside a
    |{tikzpicture}|. It will take the following \meta{path command} and
    put it inside a |{scope}| with the \meta{options} set. The
    \meta{path command} may either be a single command ended by a
    semicolon or it may contain multiple commands, but then they must be
    surrounded by curly braces.
    %
\begin{codeexample}[preamble={\usetikzlibrary{backgrounds}}]
\begin{tikzpicture}
  \node [fill=white] at (1,1) {Hello world};
  \scoped [on background layer]
    \draw (0,0) grid (3,2);
\end{tikzpicture}
\end{codeexample}
    %
\end{command}


\subsubsection{Using Scopes Inside Paths}

The |\path| command, which is described in much more detail in later sections,
also takes graphic options. These options are local to the path. Furthermore,
it is possible to create local scopes within a path simply by using curly
braces as in
%
\begin{codeexample}[]
\tikz \draw (0,0) -- (1,1)
           {[rounded corners] -- (2,0) -- (3,1)}
           -- (3,0) -- (2,1);
\end{codeexample}

Note that many options apply only to the path as a whole and cannot be scoped
in this way. For example, it is not possible to scope the |color| of the path.
See the explanations in the section on paths for more details.

Finally, certain elements that you specify in the argument to the |\path|
command also take local options. For example, a node specification takes
options. In this case, the options apply only to the node, not to the
surrounding path.


\subsection{Using Graphic Options}
\label{section-graphic-options}

\subsubsection{How Graphic Options Are Processed}

Many commands and environments of \tikzname\ accept \emph{options}. These
options are so-called \emph{key lists}. To process the options, the following
command is used, which you can also call yourself. Note that it is usually
better not to call this command directly, since this will ensure that the
effect of options are local to a well-defined scope.

\begin{command}{\tikzset\marg{options}}
    This command will process the \meta{options} using the |\pgfkeys| command,
    documented in detail in Section~\ref{section-keys}, with the default path
    set to |/tikz|. Under normal circumstances, the \meta{options} will be
    lists of comma-separated pairs of the form \meta{key}|=|\meta{value}, but
    more fancy things can happen when you use the power of the |pgfkeys|
    mechanism, see Section~\ref{section-keys} once more.

    When a pair \meta{key}|=|\meta{value} is processed, the following happens:
    %
    \begin{enumerate}
        \item If the \meta{key} is a full key (starts with a slash) it is
            handled directly as described in Section~\ref{section-keys}.
        \item Otherwise (which is usually the case), it is checked whether
            |/tikz/|\meta{key} is a key and, if so, it is executed.
        \item Otherwise, it is checked whether |/pgf/|\meta{key} is a key
            and, if so, it is executed.
        \item Otherwise, it is checked whether \meta{key} is a color and, if
            so, |color=|\meta{key} is executed.
        \item Otherwise, it is checked whether \meta{key} contains a dash
            and, if so, |arrows=|\meta{key} is executed.
        \item Otherwise, it is checked whether \meta{key} is the name of a
            shape and, if so, |shape=|\meta{key} is executed.
        \item Otherwise, an error message is printed.
    \end{enumerate}

    Note that by the above description, all keys starting with |/tikz| and also
    all keys starting with |/pgf| can be used as \meta{key}s in an
    \meta{options} list.
\end{command}


\subsubsection{Using Styles to Manage How Pictures Look}

There is a way of organizing sets of graphic options ``orthogonally'' to the
normal scoping mechanism. For example, you might wish all your ``help lines''
to be drawn in a certain way like, say, gray and thin (do \emph{not} dash them,
that distracts). For this, you can use \emph{styles}.

A style is a key that, when used, causes a set of graphic options to be
processed. Once a style has been defined, it can be used like any other key.
For example, the predefined |help lines| style, which you should use for lines
in the background like grid lines or construction lines.
%
\begin{codeexample}[]
\begin{tikzpicture}
  \draw             (0,0) grid +(2,2);
  \draw[help lines] (2,0) grid +(2,2);
\end{tikzpicture}
\end{codeexample}

Defining styles is also done using options. Suppose we wish to define a style
called |my style| and when this style is used, we want the draw color to be set
to |red| and the fill color be set to |red!20|. To achieve this, we use the
following option:
%
\begin{codeexample}[code only]
my style/.style={draw=red,fill=red!20}
\end{codeexample}

The meaning of the curious |/.style| is the following: ``The key |my style|
should not be used here but, rather, be defined. So, set up things such that
using the key |my style| will, in the following, have the same effect as if we
had written |draw=red,fill=red!20| instead.''

Returning to the help lines example, suppose we prefer blue help lines. This
could be achieved as follows:
%
\begin{codeexample}[]
\begin{tikzpicture}[help lines/.style={blue!50,very thin}]
  \draw             (0,0) grid +(2,2);
  \draw[help lines] (2,0) grid +(2,2);
\end{tikzpicture}
\end{codeexample}

Naturally, one of the main ideas behind styles is that they can be used in
different pictures. In this case, we have to use the |\tikzset| command
somewhere at the beginning.
%
\begin{codeexample}[]
\tikzset{help lines/.style={blue!50,very thin}}
% ...
\begin{tikzpicture}
  \draw             (0,0) grid +(2,2);
  \draw[help lines] (2,0) grid +(2,2);
\end{tikzpicture}
\end{codeexample}

Since styles are just special cases of |pgfkeys|'s general style facility, you
can actually do quite a bit more. Let us start with adding options to an
already existing style. This is done using |/.append style| instead of
|/.style|:
%
\begin{codeexample}[]
\begin{tikzpicture}[help lines/.append style=blue!50]
  \draw             (0,0) grid +(2,2);
  \draw[help lines] (2,0) grid +(2,2);
\end{tikzpicture}
\end{codeexample}
%
In the above example, the option |blue!50| is appended to the style
|help lines|, which now has the same effect as |black!50,very thin,blue!50|.
Note that two colors are set, so the last one will ``win''. There also exists a
handler called |/.prefix style| that adds something at the beginning of the
style.

Just as normal keys, styles can be parameterized. This means that you write
\meta{style}|=|\meta{value} when you use the style instead of just
\meta{style}. In this case, all occurrences of |#1| in \meta{style} are
replaced by \meta{value}. Here is an example that shows how this can be used.
%
\begin{codeexample}[]
\begin{tikzpicture}[outline/.style={draw=#1,thick,fill=#1!50}]
  \node [outline=red]  at (0,1) {red};
  \node [outline=blue] at (0,0) {blue};
\end{tikzpicture}
\end{codeexample}

For parameterized styles you can also set a \emph{default} value using the
|/.default| handler:
%
\begin{codeexample}[]
\begin{tikzpicture}[outline/.style={draw=#1,thick,fill=#1!50},
                    outline/.default=black]
  \node [outline]      at (0,1) {default};
  \node [outline=blue] at (0,0) {blue};
\end{tikzpicture}
\end{codeexample}

For more details on using and setting styles, see also
Section~\ref{section-keys}.
